This invention relates in general to metal stamping and, more particularly, it relates to stamping very intricate parts in sequential dies, at a high rate of speed and with high precision. The invention has particular application in the production of lead frames used in the packaging of semiconductor integrated circuits.
In co-pending U.S. application Ser. No. 842,322, filed Oct. 14, 1977 and titled "Stamped Lead Frame For Semiconductor Packages", (now U.S. Pat. No. 4,137,546, issued Jan. 30, 1979) assigned to the same assignee as the instant application, there is disclosed a 40-lead dual-in-line frame and ceramic package, called a 40-lead cer-dip, that features tabs holding at least groups of the longest leads, so that the latter are maintained in precise position during the sealing operation, wherein by means of heat and pressure the leads are embedded in glass on a ceramic substrate. After this sealing, the tabs are snapped off.
Production of such frames may be either by stamping or etching (chemical milling), but for high-volume production stamping is preferred for obvious reasons. The starting material is a strip of alloy 42, Kovar (TM) or other glass-sealing alloy. The alloy may have a stripe of vapor deposited aluminum thereon that covers the area that will ultimately form the lead tips. The first step is to punch index holes so that, in each successive step, the strip is advanced precisely one unit length. Thereafter, the frame is formed gradually in a number of sequential punch and die sets, any one set only stamping out a small number of areas. Ten or more separate stations may be required in an elaborate frame such as the 40-lead cer-dip. Finally, the lead pattern is established, with all of the leads connected at their outer ends to the surrounding frame, and to a central rectangle of metal at their inner tips. Depending on the design, tie bars may also connect the leads. The rectangle, frame and tie bars must of course be removed from the finished package. This is no problem in so-called window frame or multilayer packages, where the ceramic substrate has a central opening slightly larger than the rectangle. After the lead is embedded in one surface of such a substrate, a punch is used to remove the rectangle, frame, etc., leaving each lead electrically isolated. A solid substrate with the bonding pad for the circuit in the center is then sealed to the other surface.
In monolithic packages, however, the substrate has a central cavity or depression and punching out the center area is not possible after sealing, so this operation must be done earlier in the process. This is no problem in simple frames with a small number of leads, since the leads are large enough, and not so long, that they will not move around during sealing. In a 40-lead cer-dip or larger frame, however, the leads may be as small as 0.012 in. on 0.012 in. centers, with some leads having an overall length of more than an inch. They are so fragile that, under conditions of sealing they can and do move around enough to short one lead against another.
The break-off tabs are the solution to this problem, and the present invention is directed to the tools used to produce frames with this feature in a high-volume, high-precision stamping operation.